Kinetics mixed type inhibition

CHEMICAL KINETICS MIXED-TYPE INHIBITION MIXED VALENCY MIXING TIME... 

As for deaminase, the kinetic analysis suggests a partial mixed-type inhibition mechanism. Both the Ki value of the inhibitor and the breakdown rate of the enzyme-substrate-inhibitor complex are dependent on the chain length of the PolyP, thus suggesting that the breakdown rate of the enzyme-substrate-inhibitor complex is regulated by the binding of Polyphosphate to a specific inhibitory site (Yoshino and Murakami, 1988). More complicated interactions were observed between PolyP and two oxidases, i.e. spermidine oxidase of soybeen seedling and bovine serum amine oxidase. PolyP competitively inhibits the activities of both enzymes, but may serve as an regulator because the amino oxydases are also active with the polyamine-PolyP complexes (Di Paolo et al., 1995). 

When an inhibitor is added to the enzyme reaction, the reaction mixture may comprise more than one enzyme complex, namely ES, El, and/or ESI (Scheme 16.1) (Segel, 1987 Shou et al., 2001). Since the ES concentration ([ES]) decreases with an increase in [I], the rate of product formation (kp[ESJ) can decline (0 general kinetic model used to describe the interaction between substrate (S), inhibitor (I) and enzyme (E). Based on nature of inhibition, inhibition kinetics can be categorized to competitive, noncompetitive, uncompetitive and mixed type inhibitions. 

Fig. 3.6 Secondary plots for the determination of kinetic parameters. Cl competitive inhibition NCI non-competitive inhibition UCl uncompetitive inhibition MTl mixed-type inhibition...

Visoltricin (2-propenoic add, 3-[I-methyI-4-(3-methyI-2-buten-l-yI)-I H-imidazol-5-yI]-methyI ester) (Fig. 49.6) is an alkaloid isolated from the microfungus Fusarium tricinctum [76]. It was reported to have potent AChE-inhibituig properties with IC50 values of 2.6 x 10 M and 4.0 X 10 M in human serum and erythrocyte AQiE, respectively [77]. Kinetic studies indicated that visoltricin is a reversible inhibitor of AChE with a mixed-type inhibition in concentration-dependent manner, but independent of time. 

The PL-catechin conjugate showed greatly amplified concentration-dependent inhibition activity against bacterial collagenase (ChC) on the basis of the catechin unit, which is considered to be due to effective multivalent interaction between ChC and the catechin unit in the conjugate. The kinetic study suggests that this conjugate is a mixed-type inhibitor for ChC. Hyaluronidase is an enzyme which catalyzes hydrolysis of hyaluronic acid and is often involved in a number... 

These equilibrium-binding relationships give rise to four different kinetic responses competitive inhibition, uncompetitive inhibition, non-competitive inhibition, mixed inhibition. Details of the kinetics of these types of inhibition and how dissociation constants for the reactions can be measured are provided in Appendix 3.6. 

Studies on the inhibition of partially purified FPGS from human liver by the sulphonic acid analogues of MTX and AMT have also been reported [288]. The L-homocysteic acid analogues L-(VIII.IOO) and l-(VIII.102) had estimated K, values of 131 5 and 35 2 /iM, respectively, whereas the corresponding values for the L-cysteic acid analogues L-(VIII.lOl) and l-(VIII.103) were 173 + 6 and 66 5 pM. Interestingly, the kinetics of inhibition observed with the human enzyme were strictly competitive for l-(VIII. 100) and l-(VIII.102), but were of the mixed type for L-(VIII.lOl) and l-(VIII.103). While the latter... 

Kinetic experiments for inhibition reactions are also conducted as mentioned above for the noninhibited reaction (i.e. by varying the substrate concentration in the presence of a fixed concentration of the inhibitor) [6]. Figure 15.3, in which the data for the inhibition of the aminopeptidase-catalyzed proteolysis of methionine enkephalin by acetylated methionine enkephalin are plotted, shows an example of a mixed type of inhibition [6]. 

This article describes various approaches to inhibition of enzyme catalysis. Reversible inhibition includes competitive, uncompetitive, mixed inhibition, noncompetitive inhibition, transition state, and slow tight-binding inhibition. Irreversible inhibition approaches include affinity labeling and mechanism-based enzyme inhibition. The kinetics of the various inhibition approaches are summarized, and examples of each type of Inhibition are presented. 

In textbooks dealing with enzyme kinetics, it is customary to distinguish four types of reversible inhibitions (i) competitive (ii) noncompetitive (iii) uncompetitive and, (iv) mixed inhibition. Competitive inhibition, e.g., given by the product which retains an affinity for the active site, is very common. Non-competitive inhibition, however, is very rarely encountered, if at all. Uncompetitive inhibition, i.e. where the inhibitor binds to the enzyme-substrate complex but not to the free enzyme, occurs also quite often, as does the mixed inhibition, which is a combination of competitive and uncompetitive inhibitions. The simple Michaelis-Menten equation can still be used, but with a modified Ema, or i.e. ... 

The inhibition of certain enzymes by specific metabolites is an important element in the regulation of intermediary metabolism and most often occurs with cooperative enzymes that are regulated allosterically. Inhibition of enzymes that obey the Michaelis-Menten equation, noncooperative enzymes, is more commonly used by pharmacists to alter a patient s metabolism. Reversible inhibition of noncooperative enzymes is classified into three groups which can be distinguished kinetically and which have different mechanisms and effects when administered. The classes are called competitive, uncompetitive, and noncompetitive inhibition. Mixed inhibition also occurs. In all these types of inhibition, the inhibitor (usually a small molecule) binds reversibly and rapidly with the enzyme. 

For each of the four types of inhibition of a Michaelis-Menten enzyme [competitive, Eq. (5.25) noncompetitive and mixed Eq. (5.29) and uncompetitive, Eq. (5.32)], derive the corresponding Lineweaver-Burk equations [Eqs. (5.26), and (5.30), respectively] and draw the characteristic plots that are the basis for the rapid visnal identification of which type of inhibition apphes when analyzing enzyme kinetic data. 

FIGURE 4.10 Diagnostic plots to determine the type of inhibition occurring in a kinetic reaction. Left side, panels (a), (b), and (c) are Dixon plots representative of competitive, mixed, and uncompetitive inhibition, respectively. Right side, panels (d), (e), and (f) are Cornish-Bowden plots representative of competitive, mixed, and uncompetitive inhibition, respectively. 

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